[llvm] [llvm-objdump] Implement decoding auxiliary header for xcoff with llvm-objdump --private-headers (PR #105682)

[James Henderson via llvm-commits]

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+## Test that `the llvm-objdump --private-headers` print out the auxiliary header of XCOFF object file.
+# RUN: yaml2obj %s -o %t1
+# RUN: llvm-objdump --private-headers %t1 | \
+# RUN:   FileCheck %s --check-prefix=CHECK32 --match-full-lines --strict-whitespace
+# RUN: yaml2obj %s -DMAGIC=0x1F7 -DFLAG64=0x2 -o %t2
+# RUN: llvm-objdump --private-headers %t2 | \
+# RUN:   FileCheck %s --check-prefix=CHECK64 --match-full-lines --strict-whitespace
+
+## Test the auxiliary header of the XCOFF object file contains a partial field.
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jh7370 wrote:

You aren't testing the contents of the auxiliary header, which is what this comment is saying; you're testing the behaviour of llvm-objdump when it tries to print such an auxiliary header.
```suggestion
## Test how llvm-objdump behaves when the auxiliary header of the XCOFF object file contains a partial field.
```
Make sure to wrap the suggestion at an appropriate point.

https://github.com/llvm/llvm-project/pull/105682